This joining technique is known, for example, from DE 10 2004 056 782 A1 or DE 10 2007 049 362 A1. Substantially, in this context, the two suitably mounted and clamped parts to be joined are irradiated with a laser beam in an operating field along a weld seam to be produced, wherein the beam direction of the laser beam is controlled by a corresponding control method with control data corresponding to the weld-seam course to be produced. Furthermore, it is known here that the irradiation along the weld seam can be implemented using so-called contour welding, wherein each weld-seam position is irradiated by the laser beam once. As an alternative, a quasi-simultaneous welding can also be implemented, in which the laser beam is guided over the weld-seam course multiple times in short time intervals.
Furthermore, it is known from the named documents that the focal zone, that is, in particular, the beam diameter in the joint plane, is smaller than the width of the weld seam to be produced. In order to produce a weld seam of a corresponding width, the above-named DE 10 2004 056 782 A1 proposes moving the laser beam with its focal zone in a first movement component in a principal forward-feed direction along the track of the weld seam to be produced, and, in a second, oscillating movement component superimposed over the former, with an oscillation amplitude width, in order to cover the weld-seam width transversely to the principal forward-feed direction. In the case of a superimposition, for example, of a circular oscillating movement component over a rectilinear movement along the principal forward-feed direction, a spirally extending path of the laser focus is therefore obtained, wherein the successive spiral strokes overlap one another more or less strongly dependent upon the ratio between oscillation frequency and rate of forward-feed. Overall, through the supply of heat in the region irradiated by the laser focus in the joint plane, the thermoplastic material of at least one of the two parts to be joined is melted, and, through thermal conduction and melting of the second part to be joined, a welding is achieved between the two parts to be joined.
In terms of welding-plant technology, the welding method explained above is generally realised with a galvanometer scanner for beam deflection, through which the laser beam can be guided over a defined operating field by corresponding deflection. In this context, a so-called f-theta lens is used in the imaging optics, which has the property of focusing the beam falling through the lens to the side of the optical axis in such a manner that the focal position is disposed in the same plane as in the case of a beam which extends directly on the optical axis. Such an f-theta lens is typically constructed from a combination of two to four aspherical lenses, which are designed with a shape such that the focal-position adaptation functions well for a given wavelength.
In the case of laser welding, it is desirable for process regulation and control to couple measuring apparatus, such as a pyrometer, into the beam course. However, because of the f-theta lens, a lateral wandering apart of the weld spot and the optical axis of the pyrometer can occur if the scanner is increasingly deflected. To this extent, the detection of the weld spot by the pyrometer is no longer guaranteed.
Now, if the f-theta lens is omitted, the problem arises that, with increasing deflection from the optical axis of the laser plant, that is, with an increasing angle of incidence of the laser beam onto the joint plane, the laser beam is disposed with its focus outside the joint plane and is therefore de-focused, and the weld spot therefore becomes larger.
Furthermore, a de-focusing of the laser beam is also observed if the joint plane is displaced relative to the focal position, for example, if the two parts to be joined assume an at least slightly stepped course along the weld seam.
In fact, the de-focusing phenomena explained above are per se tolerable with regard to the energy input into the joint plane, however, with a given oscillation amplitude width, the irradiated area transversely to the principal forward-feed direction becomes larger as a result of the weld-spot widening with increasing deflection from the optical axis, so that the weld seam also becomes wider.